Current in large bodies of water, as for instance that generated by tidal variations, is a renewable source of energy which thus far has not been exploited in Norway. This is the case even though such currents are highly predictable and readily available per se along the Norwegian coast.
When currents in large bodies of water (e.g., oceans or rivers) are forced through channels or other narrow passages, the velocity of flow will increase and will to a very great extent be aligned. The current in the central region of the narrow passage will have an almost equal velocity across the whole of the cross-section in question, which means that it is advantageous to position a power generator in this region.
There are a number of known apparatus and methods for the production of energy from ocean currents.
Norwegian Patent Application 1999 1984 (Hammerfest Strøm) describes a plant for the production of electric power from ocean and river currents. The whole of the plant is located below the surface of the water and comprises a plurality of turbines having blades, a support system, a system of stays and a generator. The turbine shafts are oriented perpendicular to the direction of movement of the water, and the blades are wing-shaped so that the turbine rotates in the same direction, regardless of the direction of movement of the water. The turbine shafts are supported in a framework of buoyancy tanks, secured to the support and bearing system. The plant is built up of modules. The plant has positive buoyancy regulated by the buoyancy tanks and a system of stays secured below the surface of the water, so that the plant is held below the surface of the water by the system of stays. The plant employs conventional blades.
Danish Patent 155454 (Hans Marius Pedersen) describes a floating water current power station which consists of a ring-shaped pontoon which by means of a bitt is anchored to anchors on the bottom. All the turbines are replaceable and are arranged on a common beam and can as a unit be swung up to the surface within the area defined by the ring pontoon. The power station may move around the bitt, the upper end of which is connected to a front pontoon and the lower end of which is secured to the anchors.
U.S. Pat. No. 5,440,176 describes a submersible water turbine plant comprising turbines/generators in different combinations suspended below a submerged platform of the tension leg type.
Generators with contra-rotating rotor and stator are well known. In the field of wind energy technology coaxially contra-rotating propellers are known, inter alia, as a measure for extracting the rotational energy with which the air current is supplied when it passes the first propeller. The contra-rotating propellers may be on the same side of the generator, or on each side thereof. The propellers are either connected by means of gears to the same generator, or one propeller is connected to the rotor and the other propeller is connected to the field coils. The previously known contra-rotating propellers connected to the same generator require complex gearwheel drive and transmissions, which results in further energy loss. A contra-rotating propeller connected to the rotor and the stator respectively is much simpler from a mechanical point of view, but in the solutions known hitherto the field coils require slip rings which may render the solution more complex, in addition to there being an electrical and mechanical power loss.
In the known cases where the propellers are on each side of the generator (and the tower), it is preferable as a rule that the downstream propeller should be smaller than the upstream propeller, and set to stall at a higher wind speed.
On the production of electricity from water currents, a lowest possible propeller speed is required. The consequence is a high torque and it is very difficult to dimension a standard gear solution.
Known generator solutions usually have a constant rotational speed (pitch adjustment of the propeller) and stator that is stationary. This results in the generator housing having a very large diameter, which is a disadvantage.
U.S. Pat. No. 4,291,233 describes a wind turbine generator having oppositely rotating rotor and stator. Rotational energy from a wind-driven turbine (propeller) having a preferably horizontal shaft is transformed into rotational energy in two preferably concentric shafts, via a bevel gear drive. The propeller shaft is attached to the bevel gear drive, which engages with upper and lower pinion gears that rotate in opposite directions. The pinion gears are secured to respective shafts which are preferably concentric and arranged vertically. The two shafts—which rotate in opposite directions—are connected to a rotor and a stator respectively.
DE 43 04 577 A1 describes a wind turbine generator having two pairs of vanes connected to a rotor and a stator respectively in order to turn the pairs of vanes in opposite directions. Both pairs of vanes are arranged on the same side of the generator, and function as two contra-rotating, double-bladed propellers. This document does not disclose any further details with regard to the interaction of the stator and the rotor.
DE 196 43 362 describes a wind or turbine generator having oppositely rotating rotor and stator. A shaft 8—having a first end 10 connected to a turbine or propeller 3—is secured to a rotor 9. A stator 13 is secured to a second shaft end 11, which in turn is connected to a turbine or propeller 4. The shaft end 11 is supported on the shaft 8 via a bearing 14, so that the rotor and the stator can rotate relative to each other.
Reference is also made to an auxiliary rotor 17 and an auxiliary stator 18, where the rotor 17 is secured to the housing. Thus, the relative speed between the components 17 and 18 is half of the speed between the components 9 and 13.
The known contra-rotating generators can be divided in to two main groups:                (i) the rotor and stator are driven by the same turbine or propeller        (ii) the rotor and stator are supplied with rotational energy from their respective turbine or propeller.        
The first category of generator can be illustrated by U.S. Pat. No. 4,291,233 (described above). The disadvantage of this generator is that much energy is lost in the many transmissions that are used to contra-rotate the stator. Furthermore, this generator is bulky, and will probably be expensive both to manufacture and to maintain. Therefore, there is a need for a simplified type of generator, where the rotor and the stator are driven by the same turbine or propeller, and where a minimum of gear transmissions are used and the use of bevel gears is avoided.
The second category of generator can be illustrated by DE 196 43 362 (described above). The drawback with this generator is that it includes an outer member which does not rotate, and which is equipped with a conventional stator winding. This housing also has base lugs and takes up about half of the total torque.
The main cylinder is assumed to have magnets on both sides and is rotated by a separate shaft. Within this rotor there is another rotating stator (or rotor). The torque that arises between the innermost rotor (9) and the magnet rotor (13) does not impact on the base lugs, but is taken up in the magnetic field between the propellers. Therefore, there is a need for a simplified type of generator, where the rotor and stator are supplied with rotational energy from their respective turbine or propeller, where the base is not required to take up the torque in the generator between the rotor and the stator, and where the torque of the propellers is used in its entirety to generate electric power.